The field of the disclosure relates generally to additive manufacturing systems, and more particularly, to systems and methods for monitoring a melt pool in additive manufacturing systems.
At least some additive manufacturing systems involve the buildup of a powdered material to make a component. This method can produce complex components from expensive materials at a reduced cost and with improved manufacturing efficiency. At least some known additive manufacturing systems, such as Direct Metal Laser Melting (DMLM) systems, fabricate components using a laser device and a powder material, such as, without limitation, a powdered metal. The laser device generates a laser beam that melts the powder material in and around the area where the laser beam is incident on the powder material, resulting in a melt pool. In some known DMLM systems, component quality may be impacted by excess heat and/or variation in heat being transferred to the metal powder by the laser device within the melt pool.
In some known DMLM systems, component surface quality, particularly overhang or downward facing surfaces, is reduced due to the variation in conductive heat transfer between the powdered metal and the surrounding solid material of the component. As a result, local overheating may occur, particularly at the overhang surfaces. The melt pool produced by the laser device may become too large resulting in the melted metal spreading into the surrounding powdered metal as well as the melt pool penetrating deeper into the powder bed, pulling in additional powder into the melt pool. The increased melt pool size and depth, and the flow of molten metal may generally result in a poor surface finish of the overhang or downward facing surface. It is therefore desirable to monitor the melt pool during the build process for process control and development.
In some known additive manufacturing systems, such as DMLM systems, beam splitters are used to facilitate optical or infrared monitoring of the melt pool. Such beam splitters divide the laser beam from electromagnetic radiation generated by the melt pool, and thereby allow a single scanning device to scan the laser across the build surface and to reflect light generated by the melt pool to optical detectors for monitoring the melt pool. In other words, such systems allow optical detectors to “look” coaxially along the laser beam to monitor the melt pool during the manufacturing process. However, such beam splitters have reflective coatings or layers that absorb a small percentage of light from the laser beam, causing the beam splitters to heat up and undergo thermal expansion. This phenomenon, known as “thermal lensing,” can cause the shape and the refractive index of the beam splitter to change, which can result in distortion of the laser beam profile, the laser beam spot size, and the melt pool image detected by the system.
As the demand for higher-throughput systems increases, higher power lasers are used to increase the build speed of additive manufacturing systems. Higher power lasers exacerbate the effects of thermal lensing, which can negatively affect the build process and the ability to accurately monitor the melt pool.